[R-sig-Geo] SpatialPolygons decomposition...

[Jim Burke]

Hi Peter,

I think I am three days ahead of you on the learning curve. So come join 
me! Below is a cumulation of suggestions from Rodger Bivand.

Perhaps the code below may help. Start out with your own SpatialPolygon. 
Change it to a dataframe. Then do something data like to the dataframe. 
Then coerce back to a SpatialPolygon dataframe. Then dump the 
SpatialPolygon in various ways. Or you could simply dump the 
SpatialPolygon. 

# the following code takes an sp to a df to add the two columns
# then the df is coerced back into an sp. This solves a merge
# issue when merging an sp and df together. R thinks the result
# should be a data.frame so good bye SpatialPolygons   

    tx2_df <- as(tx2_sp, "data.frame")   #make a sp into a df

    tx2_df1      <- merge(tx2_df, votes2_df, sort=FALSE, by.x="PCT",
    by.y="PCT", all.x=TRUE, all.y=TRUE)
    remove(tx2_df)
    remove(votes2_df) 

    # notice that the data frame row IDs we print are sequential
    # 1,2,3,4... and not proper precinct names like 1234....
    rownames(as(tx2_df1, "data.frame"))  #show us the row IDs

    # key here is match.ID = FALSE so that it does not try to take
    # the 1,2,3 data frame sequence numbers and think they are IDs. 
    # both sp and df must have rows aligned the same.
    tx3_sp <- SpatialPolygonsDataFrame(as(tx2_sp,"SpatialPolygons"),
              data=tx2_df1, match.ID = FALSE)
    remove(tx2_sp)
    remove(tx2_df1)

    # debug tx3_sp a little, lets make sure its a SpatialPologonsDataFrame!
    sapply(slot(tx3_sp, "polygons"), function(x) slot(x, "ID")) #what 
are row "ID"s?
    str(as(tx3_sp, "data.frame"))         #show representation of variables
    (str(tx3_sp))                         #shows representation of 
geometries too.
    names(tx3_sp)                         #nice but lengthy  

Let us know if this helps you any.

Good luck,
Jim Burke



Peter S. Hayes wrote:
> Hi All,
>
> I'm trying to teach a spatial analysis class using R as a means of 
> learning some details... I'm not an expert in R myself, but am 
> learning it while using it as a 'tool' for lessons in the class...
>
> In getting the students familiar with some of the SP classes, we began 
> looking at the class components and manipulating some of the 
> components (such as translating by modifying coordinates..).
>
> Is there a means of decomposing SpatialPolygons to access the list of 
> Polygons and contained classes? The help files (for example, 
> polygons()) appear to hint so, but not function so... the 
> SpatialPolygons isn't quite a traditional R dataframe and won't 
> flatten with a call to as.data.frame() to allow access to the 
> individual slots... but there must be a means of doing that...
>
> Also, I have "Applied Spatial Data Analysis with R" as one of the 
> texts... any thoughts for more references... especially for 
> non-programmers: I've some software experience (C/C++...) but most of 
> the students are environmental study, environmental science, or 
> biology students and this is one of their first experiences with 
> anything having a command line. We've been taking things slow, but R 
> is still a bit cryptic... any thoughts on that would be appreciated!
>
> Thank you for all!
>
>
> Pete
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at stat.math.ethz.ch
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
>